Great gift/starter and to redo it with dad would be priceless memory. Shooting a .410 at cans, clay pigeons proped up and would be great for his first squirrel hunt. It won't pound him like a 20ga and sure is a lot lighter to carry. Some people down play the .410 but it is a very capable round and never let your son forget that.
.410's are great for that. I shot a .410 squirrel hunting fro years, when I got my .20ga 1100 I was very surprised it kicked less than the .410. The pump is a tad different though